Drive By Wire

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The new 2005 Ford Escape Hybrid is one of a new breed of vehicle, using both gasoline and electricity for propulsion. Why two motors, you may ask? While the new Hummer H2 chugs along at a puny 10 to 13 mpg, and the conventional version of the Ford Escape gets 21, the Escape Hybrid will get 36 mpg in city driving. And it’s all made possible by computers.

First a little background: The term fly by wire describes modern aircraft such as the Stealth Bomber, which are so inherently unstable that only a computer can make decisions quickly enough to keep them airborne. As a pilot moves the controls, adjustments are transmitted to computers, which fine-tune the flight surfaces to achieve the desired effect. The Escape Hybrid is a drive-by-wire vehicle, not because it’s unstable, but because computers make hundreds of decisions every second about the car’s systems in order to achieve maximum efficiency.

The Escape’s seven separate computers are connected by a CAN (Controller Area Network), a serial network standard in automobiles. Most CANs transfer data at low speeds, typically 10 to 125 Kbps, and some less time-critical information in this vehicle is carried on a 125-Kbps bus. But the Escape also needs to move a lot of critical info from place to place, so it runs a second bus at a much higher speeda whopping 500 Kbps. Three on-board computers operate more or less independently, using the CAN to transmit diagnostic information from other modules. The Instrument Cluster processes this info and can trigger messages on a dashboard display. This module can notify the driver of a range of information, including instantaneous fuel economy figures and low fuel alerts. Steering is power-assisted with an electric motor; an Electric Power Steering Computer takes input from the steering wheel and the angle and rotation speed of the front tires to determine how much assistance to give. And the Intelligent Torque Coupling Controller helps prevent wheels from slipping in four-wheel drive mode by adjusting the torque between the front and rear axles to maximize traction.

Four other modules work in close concert with each other. The Powertrain Control Module (PCM) is the command center for the propulsion system. It processes the pressure of the driver’s foot against the accelerator and controls the electronic throttle on the gasoline engine. It also receives input from other devicesincluding the gearshift and the brake pedalto determine how much torque is requested from the gasoline engine and the electric motor. A Motorola PowerPC microprocessor does most of the PCM’s thinking, but the PCM also houses many satellite processors for safety monitoring, electronic throttle controls, and diagnostics. These range from off-the-shelf Motorola HC-12 chips to custom processors.

The Transmission Control Module (TCM) interprets messages from the PCM and automatically starts or shuts down the gas engine as needed. That’s right, the engine starts and shuts off automatically while you’re driving, depending on how much power is required. The TCM also controls the electric motor, either providing torque or recharging the battery through regenerative braking, as needed. The Traction Battery Control Module monitors the state of charge of a 330-volt NiMH battery pack. It can request cooling from a special chiller designed solely to keep the battery within an optimum temperature range, so that it doesn’t overheat and lose power. The Brake System Control Module controls the brake-by-wire system, which couples the regenerative breaking with a traditional friction-braking system. This module also distributes braking requests so that the feel of the brake pedal is constant over any combination of braking methods.

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